Excerpt from a Star K article about shavers:
Hagaon R' Moshe Feinstein זצ"ל was of the opinion that the גמרא 's
statement - איזהו גילוח שיש בו השחתה הוי אומר זה is teaching us that
only the תער , the straight-edged razor, is the Torah's forbidden form
of השחתה and גילוח . Any other method of השחתה and גילוח would be
permitted. Based on this fundamental understanding of the גמרא ,
electric shavers would be permitted even if the shaver removed facial
hair beneath the skin, since the shaver works in a scissor-like and
not razor-like action.
However, due to the increased sharpness of the blades of the rotating
heads it is conceivable that the blades could cut the beard hairs off
before the combined cutting of blade and screen. Therefore, even
according to the Poskim that permit electric shavers, this shaver
would be problematic, because it would be cutting like a תער , which
is אסור . Can one practically determine whether the shaver in
question is a kosher model?
Hagaon R' Moshe Feinstein זצ"ל , who permitted the use of electric
shavers used a criteria similar to the shochet who would demonstrate
how sharp his חלף (shechita knife) was. To show the sharp edge of his
knife, a shochet would take a hair from his beard, and holding the
hair in one hand the shochet would see whether the חלף cut the
dangling hair. If the hair was severed, the shochet's steel passed
So too, in a similar manner, R' Moshe would often test the sharpness
of an electric shaver's blades. He would take a beard hair and test
the blades! The shaver would be acceptable if the hair was held taut
and was not split in half.
It appears that the pop-up trimmer is no sharper than the rotary blades. IMO, they are duller than the blades, actually. So, it seems that it should be OK.
If @Shokhet is reading this, I wonder if he has performed Rav Feinstein's test using his Shochet knife?